For 64-bit processors, lwsync is the recommended method of store/store
ordering on caching enabled memory.  For those subarchs which have
lwsync, use it rather than eieio for smp_wmb.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NPaul Mackerras <paulus@samba.org>